Many times there is a fit issue when you have a pinned barrel receiver and are fitting a non-original mag to it. Even if it is a Chinese mag. If the mag is off of a threaded barrel receiver you may have to file away some of the metal on the tang at the front of the mag where it fits into the slot on the bottom of the rear sight housing. File a little at a time until the back of the mag inner housing just fits up into place along the bottom side of the receiver magwell area. I usually do this with the stock off as it is easier to do your test fits between filing and can better see where the interference is. Also, something to remember, when fitting any firearm part, a little is a lot. Better to file and check fit 4 times before it fits right than to grind a bunch off and end up with a piece of scrap metal.
I will throw this out there as well, just in case, The trigger assembly has to be removed for the mag inner housing to fit up into place. If you haven't removed the trigger group the mag will never go in. I only say this because you didn't specifically mention that you removed it and I don't know your working knowledge level on the SKS. (I figure better safe than sorry)